1. Field of the Invention
This invention relates to a membrane switch utilized as e.g., a keyboard of a note type personal computer, and particularly relates to a waterproof membrane switch in which no water droplet enters the interior of the membrane switch even when the membrane switch is wet with water.
2. Description of the Background Art
A membrane switch having a laminating structure of flexible membrane sheets is arranged as a keyboard for inputting data in a portable device such as a portable note type personal computer.
As shown in FIG. 17, this conventional membrane switch 100 has an upper membrane sheet 101, a spacer sheet 102 and a lower membrane sheet 103 each constructed by a flexible polyester film having about 0.1 mm in thickness. A pressure sensitive adhesive is coated between these sheets so that an adhesive material layer 104 is interposed between these sheets and the three sheets 101, 102 and 103 are laminated with each other.
Plural through holes 106 are bored in the spacer sheet 102. Upper and lower portions of these through holes 106 are covered with the upper membrane sheet 101 and the lower membrane sheet 103 so that plural switch storing chambers 105 are formed. A movable electrode 101a is printed on a lower face of the upper membrane sheet 101 facing each of the switch storing chambers 105. Further, a fixing electrode 103a constituting a pair together with the movable electrode 101a is printed on an upper face of the lower membrane sheet 103 opposed to this movable electrode 101a.
Each movable electrode 101a and each fixing electrode 103a are connected to the exterior of the switch by an unillustrated lead portion. When the upper membrane sheet 101 above the switch storing chamber 105 is pressed, the movable electrode 101a comes in contact with the fixing electrode 103a and switch operating data of this switch storing chamber 105 are outputted to the exterior.
In this pressing operation, no air within the switch storing chamber 105 flows out in a sealing state in which the switch storing chamber 105 is interrupted from the exterior. Therefore, no movable electrode 101a reaches the fixing electrode 103a unless the upper membrane sheet 101 is strongly pressed. Accordingly, no membrane switch is easily operated. In contrast to this, when an external atmospheric pressure is increased in comparison with the atmospheric pressure within the switch storing chamber 105, there is a case in which the electrodes 101a and 103a come in contact with each other without performing any pressing operation, thereby causing an error in operation of the switch.
Therefore, in the conventional membrane switch 100, a concave groove is formed in the spacer sheet 102. Further, as shown in FIG. 18, a gas permeable passage 107 is formed by arranging an uncoating area of the adhesive material around the switch storing chamber 105 so that the switch storing chamber 105 is communicated with the exterior.
However, when the membrane switch 100 is wet with water, there is a case in which a water droplet enters the switch storing chamber 105 from this gas permeable passage 107 and is attached to the electrodes 101a, 103a. When an operating electric current of the switch flows through the electrodes 101a, 103a in this attaching state of the water droplet, a chemical change is caused by silver migration, etc. and causes insulation and contact defects, etc.
In recent years, carrying opportunities of the note type personal computer are increased and this personal computer is often exposed to the rain. Further, dew condensation tends to be caused by a change in temperature to move the personal computer to a place different in using environment. Accordingly, a more perfect waterproof property is required in the membrane switch. However, since it is necessary to form the gas permeable passage 107 as mentioned above, no sufficient waterproof performance is obtained.